Recently, secondary batteries are widely used as power sources for mobile devices such as mobile phones, notebook computers, and camcorders. In particular, the use of lithium secondary batteries has rapidly increased due to advantages that include a high operating voltage and a high energy density per unit weight.
The lithium secondary battery may be classified as a lithium ion battery, a lithium ion polymer battery (LiPB), or a lithium polymer battery depending on the configuration of an electrode and an electrolyte. Among the types of lithium secondary battery, lithium ion polymer batteries, that are less likely to leak an electrolyte and are more easily manufactured, are increasingly used.
The LiPB has a structure in which an electrode assembly in which electrodes (a positive electrode and a negative electrode) and a separator are thermally fused is impregnated with an electrolyte, and is mainly used in a form in which the electrode assembly is sealed in a pouch-shaped case of an aluminum laminate sheet. Therefore, the LiPB is often referred to as a pouch type battery.
FIG. 1A schematically shows a typical structure of a typical secondary battery including a stacked/folded type electrode assembly, and FIG. 1B schematically shows a process of manufacturing the electrode assembly of FIG. 1A.
Referring to FIG. 1A, a secondary battery 10 has a structure in which an electrode assembly 30, which is composed of a positive electrode, a negative electrode, and a separator disposed therebetween, is built into a pouch type battery case 20, and positive electrode and negative electrode tabs 31 and 32 thereof are welded to two electrode leads 40 and 41 and exposed to the outside of the battery case 20.
The battery case 20 is made of a soft packaging material such as an aluminum laminate sheet, and includes a case body 21 including a concave shaped storage part 23 on which the electrode assembly 30 can be seated, and a lid 22 having one side coupled to the case body 21.
The electrode assembly 30 used in the secondary battery 10 has the stacked/folded structure shown in FIG. 1A. In the electrode assembly 30, a plurality of positive electrode tabs 31 and a plurality of negative electrode tabs 32 are welded to the electrode leads 40 and 41, respectively. Insulating films 51 and 52 are attached to the electrode leads 40 and 41, respectively.
Referring to FIG. 1B, the electrode assembly 30 is manufactured by arranging a plurality of unit cells 61 and 62 on a separator sheet 50 and sequentially winding the separator sheet 50 on the unit cells 61 and 62.
In particular, when an end part 70 (a so-called “zero separator”) of the separator sheet 50 covering the first unit cell 61 falls and turns over after a first winding process of the first unit cell 61, a surface of the first unit cell 61 and a surface of the second unit cell 62 are brought into contact with each other, and thus a problem which causes a low voltage issue often occurs.
Accordingly, there is a significant need for technology capable of minimizing separation of a separator sheet from a unit cell during a manufacturing process for an electrode assembly by fundamentally eliminating these problems.